Slag has many commercial uses, and is rarely discarded. It is often reprocessed to separate any other metals that it may contain. The remnants of this recovery can be used in railroad track ballast, and as fertilizer. It has been used as a road base material and as a cheap and durable means of roughening sloping faces of seawalls to progressively arrest the movement of waves. Blocks of slag have been used in the construction of retaining walls and foundations.
What was once an unwanted by-product of the steel making process, can now be recycled and used in the manufacture of high performance concretes. When iron ore is heated in a blast furnace, the impurities or ‘slag’, which include large quantities of calcium and silica, become molten and are separated from the raw iron.
As the slag is channeled out of the furnace, thousands of gallons of water are poured over it. This rapid cooling, often from a temperature of around 2,600° C., is the start of the granulating process. This process causes several chemical reactions to take place within the material, and gives the slag its cementitious properties.
The water carries the slag in its slurry format to a large agitation tank, from where it is pumped along a piping system into a number of gravel based filter beds. The filter beds then retain the slag granules, while the water filters away and is returned to the system.
When the filtering process is complete, the remaining slag granules, which now give the appearance of coarse beach sand, can be scooped out of the filter bed and transferred to the grinding facility where they are ground into particles that are finer than Portland cement
This previously unwanted recycled product is used in the manufacture of high performance concretes, especially those used in the construction of bridges and coastal features, where its low permeability and greater resistance to chlorides and sulfates can help to reduce corrosive action and deterioration of the structure.
An interlocking paver is a pre-cast piece of concrete or brick commonly used as an alternative to plain concrete or asphalt or other paving materials. Pavers can be assembled to cover walkways, patios, pool decks and driveways and airport or loading docks. Interlocking pavers are available in a wide range of shapes such as rectangular, hexagonal, etc. and each allows them to be jointed fittingly to create a paving surface. The advantage of using interlocking pavers over the asphalt and poured concrete are high compressive strengths which can reach 7000+psi, pleasant look, time saving, easy removal and relaying etc.
There are quite a few interlocking pavers that are available in the market. They are in different shapes, sizes and made of different materials. Common building materials are concrete and clay; and by adapting different manufacturing methods, pavers of various physical properties can be achieved. For example, pressing the dry concrete-mix into molds rather than pouring a wetter mix allows for their 8000 psi compressive strength, making concrete paving stones a more durable choice than clay bricks or poured-in-place concrete. Clay pavers have an advantage with resistance to fading from the sun and deterioration from long term exposure to the elements. Because clay pavers are fired, the pores of the paver are at least partially vitrified closed, therefore creating an almost non permeable surface. Their main disadvantage is choice of color spectrum. They are a natural material so colors are limited. Concrete also has a limitless color spectrum when starting with white Portland cement and pigments.
Installation of interlocking pavers starts with a compacted stone sub-base and a leveling bed of sand, pavers of desirable size, shape and material. Instead of connecting the pavers by pouring grout between the joints, as one would with tiles, sand particles are spread over the pavers and tamped down. The sand stabilizes the interlocking pavers, yet allows for some flexibility. This type of pavement will absorb stress such as small earthquakes, freezes and thaws, and slight ground erosion by shifting each paver slightly. Therefore, they are less likely to crack or buckle like poured cement.
Due to the increasing environment concerns, however, there has been a upward demand on permeable paving materials, which provide water permeable properties as well as strength for use as paving materials. Permeable paving materials allow moisture to filter through and replenish underground water tables and other water sources. It also helps to drain water into the ground and relieve stress on over taxed storm water systems during high rain conditions.
In the past, concrete pavers have contributed to the LEED (“Green Building”) rating system. Originally developed for the U.S. Department of Energy and standing for Leadership in Energy and Environmental Design, LEED is growing in use by design professionals in response to federal, state, and local government agencies, and by private developers. LEED uses a point rating system to recognize sustainable site and building design. Depending upon geographical location due to varying enabling legislation and practice in the different states, complying with the rating system is voluntary and it aims to improve environmental and economic performance of buildings and sites. Developed by consensus with the participation of many organizations, the rating system and certification program based on providing evidence of compliance to the rating system is administered by the U.S. Green Building Council. A complete description and downloads can be found on the Internet at www.usgbc.org/LEED.
Importantly, concrete pavers and permeable interlocking concrete pavers can earn points or “credits” in the LEED rating system. Credits are earned under several categories of use including stormwater management, local/regional materials, and exterior design to reduce heat islands. For stormwater management, Credit 6.1 (1 point) can be earned for building sites where the existing impervious area is greater than 50%. Permeable interlocking concrete pavement can meet this requirement. In some urbanized areas with this extent of impervious cover, permeable interlocking concrete pavement may be more cost-effective than separate water detention and/or retention facilities due to space and configuration constraints. The LEED requirement is that runoff rate and quantity be reduced by at least 25%. In the past, permeable interlocking concrete pavements have been able to reduce runoff to zero for the most frequent storms.
Credit 6.2 provides 1 point for treatment systems designed to remove 80% of the average annual post development total suspended solids (TSS), and 40% of the average annual post development total phosphorus (TP). The ability of permeable interlocking concrete pavements to reduce these pollutants is typically greater than these percentages according to references in the Interlocking Concrete Pavement Institute's manual, Permeable Interlocking Concrete Pavements-Selection, Design, Construction, Maintenance. The ICPI manual references studies on infiltration trenches (similar to permeable pavement bases) and porous pavements with reductions in TSS as high as 95% and TP as much as 70%.
Another source of credit is designated as Credit 5 (1 to 2 points), local regional materials: specify a minimum of 20% of building materials that are manufactured regionally within a radius of 800 km (500 miles). An additional point is earned if 50% of the regionally manufactured materials are extracted, harvested or recovered within this same radius. Most interlocking concrete pavers and permeable pavers will be manufactured within this distance from the project site.
Yet another Credit is 7.1 (1 point), landscape and exterior design to reduce heat islands. An option for meeting this requirement is to use light colored/high albedo materials with a reflectance of at least 0.3 for 30% of the sites non-roof impervious surfaces, i.e., pavements. Concrete paving units can be manufactured in practically any color, so they can be tailored to register an albedo of at least 0.3.
Albedo is defined as the ratio of outbound or reflected solar radiation to inbound radiation. It is measured with a pyranometer. A pyranometer is a type of actinometer used to measure broadband solar irradiance on a planar surface and is a sensor that is designed to measure the solar radiation flux density in watts per square meter from a field of view of 180 degrees. The name pyranometer stems from Greek, “pyr—???” meaning “fire” and “ano—???” meaning “above, sky”. A typical pyranometer does not require any power to operate. Long-term measurements should be done with two pyranometers rather than one to better understand and compare diurnal changes in the radiation flux of pavements.
U.S. Pat. No. 6,419,740, issued Jul. 16, 2002 to Kinari et al. teaches a water-permeable solid material which can be used as paving materials. However, the materials are not and cannot be made into paver form. U.S. Pat. No. 6,824,605, issue Nov. 30, 2004 to De Buen-Unna, et al. also teaches ecological permeable concretes with high compression, bending and abrasion resistance for paving purposes but again, the materials are not made into a paver form.